The helicopter rotor drag dampers are necessary systems for ensuring the stability of the rotor. The operational performance levels in terms of stiffness and of damping must be guaranteed to ensure the correct operation of the helicopter, while limiting, for example, risks of resonance on the ground. Such devices are described in the patents U.S. Pat. No. 3,758,230, FR 2592696 and FR 2 818 717.
In service, the dampers, and in particular the drag dampers, are subjected to severe static and dynamic loadings which progressively lead to a degradation of their operational performance levels (defined in stiffness and damping terms).
One possible solution for monitoring in service the performance levels of the dampers would be to regularly measure their stiffnesses and damping on a test bench. However, this operation requires the dampers to be dismantled and reassembled, which is incompatible with the operational constraints of the device on which they are mounted (trucks, oil platforms, wind turbines, helicopters, etc.).
The maintenance monitoring is therefore done, currently, without dismantling the dampers and there is only a regular visual inspection of the drag dampers to assess their state of health. The absence of visual defects is interpreted as a sign that the part still has its operational characteristics, whereas the appearance of signs of fatigue such as surface cracking of the elastomer is considered as an alert threshold leading to the removal of the dampers and the mounting of new parts in order to avoid any risk of failure in operation. In many cases, this leads to the scrapping of products whose operational performance levels may still conform to the requirements.
Parallel to this visual maintenance monitoring, numerous procedures provide for the automatic replacement of the dampers after a determined flight time. Given the manufacturing quality of the modern dampers, these procedures also lead to the scrapping of products whose operational performance levels may still conform to the requirements.
The visual maintenance monitoring is only a qualitative inspection of the state of the damper, but which does not make it possible to know the mechanical stresses which have been applied to the drag damper during its use on the ground, during pre-flight checks (almost static mechanical stresses), and during its use in flight (take-off, flying, landing; dynamic mechanical stresses).
It has already been proposed to equip certain helicopter controls with devices making it possible to ensure the acquisition and the transmission of the measurements of load passing through these controls. For example, the U.S. Pat. No. 7,719,416 describes a device for acquiring and transmitting measurements of load passing through a pitch control of a helicopter. This embodiment describes the integration, in a helicopter part, a pitch control, of an embedded measurement system that is autonomous energy-wise and communicates with a base station to transmit recorded data. The data, essentially consisting of the measurement of the loads applied to this helicopter pitch control, can serve to quantify the state of accumulated damage to this structural part or its remaining potential lifetime, via an analysis of the aggregate damage induced by the loads withstood by this structural part.
However, such an analysis relying on the aggregate of the loads does not make it possible to accurately predict the state of fatigue or of degradation of a hydraulic damper (that is to say whose vibration absorbing element consists of a liquid or a fluid), of an elastomer damper (that is to say, whose vibration absorbing element consists of an elastomer) or of an elasto-hydraulic damper (that is to say, whose vibration absorbing element consists of a combination of a liquid and of an elastomer). In effect, the laws of fatigue behavior of the vibration absorbing elements, in particular comprising an elastomer material, do not depend only on the aggregate damage based on loads but also on the entire history of the sequencing of the stresses, notably of the displacements applied.
The patents FR 2 867 152 and FR 2 961 333 describe a methodology making it possible to assess the operation of a damper, that does not require a dismantling of said damper, by analyzing only the relative displacement of the two frames forming the damper at the drag frequency of the rotor in dynamic operation.
However, neither of the patents FR 2 867 152 and FR 2 961 333 describes a device capable of implementing the method that they describe and of accessing the measurement of relative displacement of the two frames. Furthermore, the single measurement of relative displacement of the two frames does not make it possible to ensure a reliability necessary for the early detection of signs of degradation to trigger a programmed maintenance.